Microwave dielectric ceramics based on GdTiNb,-,.Ta,O6 and Sml _.,Y,TiTa06 have been prepared by conventional solid state
method . The GdTiTaO6 and SmTiTaO6 have aeschenite structure with positive rr and GdTiNbO6 and YTiTaO6 have euxenite
structure with negative rr. The rr of the ceramics has been tuned by preparing solid solution phases between the aeschynites and
euxenites for a possible zero rr material . It is observed that GdTiNbt_YTa.,O6 undergoes a phase transition from aeschynite to
euxenite when x=0.75 and in Sml-,YxTiTa06 for x= 0.73. The microwave dielectric properties change abruptly near the transition
region . The rr value approaches zero near the phase transition region while the samples have poor sinterability and poor quality
factor . The unloaded quality factor, dielectric constant and the sign of rr of the solid solution phases are found to depend on the
average ionic radius of the rare earth ion in RE ,-5RE',TiTaO6. The boundary of the euxenite-aeschynite phase transition occurs at
an average ( RE) ionic radius of 0.915 A in Sm,_, Y,.TiTaO6 solid solution phases